1. Field of the Invention
The present invention relates to electrical interfaces and particularly to thermally conductive adhesive interface 17 interfaces for use in a variety of electronic products.
2. Description of Related Art
Integrated circuit ("IC") chips have steadily become more powerful while being compacted into smaller and smaller packages. When compared to previous integrated circuit chips, this trend produces integrated chips which are significantly denser and which perform many more functions in a given period of time--resulting in an increase in the current they use. Consequently, smaller and faster chips tend to run significantly hotter than previous products.
As a result, heat management in electronic products has become a chief concern in product design. Reliability of electronic circuits tends to be tied to proper matches in the coefficients of expansion of various electronic components. As the temperature rises, mismatches in the coefficients of expansion cause stresses to develop between adjoining members. Under these circumstances, any increase in operating temperature will have a negative effect on reliability.
In an effort to control heat better, the use of various heat sinks is now a central focus in electronic equipment design. Examples of common heat sinks employed today include: IBM Thermal Conductive Modules (ITCM); Mitsubishi High Thermal Conduction Modules (HTCM); Hitachi SiC Heat Sink; Fujitsu FACOM VP2000 Cooling Mechanism; metal plates of copper, aluminum, etc.
In order to mate IC chips to heat sinks successfully, an interface which is elastic or otherwise conformable is preferred so as to ease installation and to minimize the effect of expansion and contraction between electronic components. Air gaps formed from inapt installation of a chip to a heat sink, and/or expansion and contraction cycles during operation, can greatly impede the flow of heat from the device. Conformability becomes especially important when the tolerances on the heat sink and chip tilt (in the case of flip chips) become large.
Typically, thermal greases or thermally conductive thermosetting materials are used to take up tolerances between electronic components. See, e.g., U.S. Pat. No. 5,028,984 to Ameen et al. While such materials may work well for some applications, they continue to have a number of drawbacks. These materials tend to be hard to control and are prone to contaminating components of the electronic device. For instance, care must be taken when using these materials to prevent unwanted contamination of solder joints and, in the case of electrically conductive thermoset resins, unwanted contamination of adjacent conductors. In practice, this usually results in a significant amount of wasted material. Additionally, clean up of such materials often requires the use of either unsafe or environmentally undesirable solvents.
In U.S. Pat. No. 5,137,283 to Giarusso et al. a gasket-type material is disclosed comprising a thin-film surrounding a meltable metal core. In operation, the gasket is installed as an interface and its temperature is increased to melt the metal core and allow it to conform to the component parts. Unfortunately, this construction is believed to be ineffective in avoiding air gaps that can form during normal thermal cycling of the device. Further, as is a common problem with solid gasket materials in general, it is believed that this device may experience limited compressibility, requiring either application of excessive pressure to the mating surfaces, or use of unacceptably thick sections of the gasket.
In U.S. Pat. No. 5,060,114 to Feinberg et al., conformability is sought by curing a metal or metal oxide filled silicone around the component to be cooled. Although this method may be successful, it is believed to be unduly complicated, costly, and time consuming for practical widespread use.
In addition, with most thermoset resins, and with gaskets employing thermally conductive particles as a filler, there are additional constraints in successful heat dissipation. In order to overcome thermal insulation between particles, it is often necessary to apply substantial pressure to the interface in order to urge the thermally conductive particles into direct contact with one another to produce the necessary amount of conduction through the material. This often requires unacceptable compressive force for integrated circuits to produce a viable thermally conductive interface.
As a result, most commercially available products can produce a conductivity in the range of only about 1.8 W/M .degree.K (for greases) to 2.2 W/M .degree.K (for epoxies). Even the most advanced (and expensive) materials, such as silver filled epoxies, can achieve a conductivity in the range of 3-4 W/M .degree.K. Easily handled materials, such as self-adhesive materials available from Chomerics, Inc., Woburn, Mass., under the trademark CHO-THERM Thermal Interface Materials, and from The Bergquist Company, Minneapolis, Minn., under the trademark SIL-PAD Thermal Management Materials, can typically achieve a conductivity of only about 0.37-1.3 W/M .degree.K and 0.6-1.5 W/M .degree.K, respectively. Although these commercial materials can produce better conductivities at high mounting pressures, they deliver poor conductivity at very low mounting pressures (e.g., pressures below 2-3 lbs/in.sup.2).
A number of other materials have been developed suitable for use in electrical circuit board construction centered around use of polytetrafluoroethylene (PTFE), and in many cases expanded PTFE as is taught in U.S. Pat. No. 3,953,566 to Gore. U.S. Pat. No. 4,985,296 to Mortimer teaches the use of a PTFE highly filled with inorganic filler that is between 0.1 and 5.0 mil thick and substantially pin hole free. This material is particularly suitable for use as an electrically or thermally conductive layer in printed circuit boards and the like. However, the process of producing this material requires densification of the membrane, significantly reducing its conformability. U.S. Pat. No. 4,996,097 to Fischer teaches similar technology useful for a thin capacitive layer in a printed wiring board (PWB). U.S. Pat. No. 4,518,737 to Traut teaches an extruded composite tape of ceramic filler and PTFE useful for its high dielectric constant. With each of these products the method for making is by bonding the PTFE in the composites at so-called "sintering" temperatures (i.e., at very high temperatures and/or pressures). These composites have not gained widespread use because of their difficult processing. This can be very inconvenient, and often impossible to accomplish, especially for many adhesive applications where materials being bonded cannot withstand the necessary temperatures and pressures.
Japanese laid-open patent application 61-40328 to S. Hamasaki, et al. teaches impregnating a silicone rubber imbibed within a porous expanded PTFE structure for use as a thin electrical insulator with thickness no greater than 50 .mu.m. A solution of silicone rubber is imbibed into the porous structure of expanded PTFE, which renders the product transparent (free of filler). The final product is then cured. In an attempt to reinforce this structure, H. Kato, et al. teaches in Japanese laid-open patent 62-100539, a silicone rubber article which is made by first incorporating a ceramic into a dispersion of PTFE, thus collecting the filler at the nodes of the node-and-fibril structure, then imbibing the silicone resin into said fibrillated structure as described above. In both of these instances, the final product is a rubber-like cured sheet.
In a similar fashion, M. Hateyama, et al., in British patent 2,195,269B (EP-0248617B1), describes an article and process of imbibing expanded PTFE with a thermosetting resin which is useful as a substrate for a PWB. Unfortunately, previous attempts at this approach have been largely unsuccessful because high degrees of ceramic loading with the addition of ceramic filler tends to weaken the node and fibril structure.
Other problems experienced by many commercially available filled thermoset resins include: inadequate conformability (i.e., excessive compressive force required to get higher thermal conductivity); high flexural modulus after curing-resulting in substantial stress upon devices during thermal cycling; a lack of "compliance," resulting in stress fractures if the resin is flexed longitudinally after curing; long curing times; and difficulty in manufacturing in high volumes.
Another property which is sought but not yet available is a convenient and effective method of supplying an adhesive for use in circuit board construction. Presently available products attempting to provide these properties tend to be non-conformable, overly brittle, or difficult to process.
Accordingly, it is a primary purpose of the present invention to provide a thermally conductive interface which delivers relatively even heat dissipation and reduces the negative impact of flex and fatigue.
It is another purpose of the present invention to provide a thermally conductive interface that simultaneously provides adhesive for the fabrication of an integrated electronic package.
It is yet another purpose of the present invention to provide a thermally conductive interface which is conformable to provide a good fit between component parts without requiring undue compressive force to achieve the desired amount of thermal conductivity.
It is still another purpose of the present invention to provide a thermally conductive interface which is compliant, allowing the material to be more forgiving to longitudinal stresses.
It is a further purpose of the present invention to provide a film adhesive that is easy to use and contributes little or no contamination of adjacent areas. These and other purposes of the present invention will become evident from review of the following specification.